Apparatus and method for coating a web

ABSTRACT

The invention provides a method and an apparatus for coating a moving web by passing the latter through a narrow die opening while maintaining a free flowing coating material between the die wall and the web surface being coated. The coating material wets both said die wall and the web surface. As the web passes through the die the coating material between the web surface and the die wall is sheared and a coating of same is applied to the web surface. The coated web is thereafter passed through a drying zone to effect the required degree of drying.

United States Patent Gunning 1 Sept. 23, 1975 APPARATUS AND METHOD FORCOATING [56] References Cited A WEB UNITED STATES PATENTS Inventor: JGunning, Oakville, Canada 2,766,721 10/1956 Paquette 118/413 x [73]Assignee: Abitibi Paper Company Ltd.,

' Toronto, Can d Primary Examiner-Michael R. Lusignan A F K Filed: Nov.1973 ttorney, Agent, or zrm Spencer & aye

Related Application Data The invention provides a method and anapparatus for [63] Continuation of Ser. No. 187,785, Oct. 8, 1971,coating a moving web by passing the latter through a abandonednarrow dieopening while maintaining a free flowing i coating material between thedie wall and the web sur- F olelgn Appllcatlon y Data face being coated.The coating material wets both said Oct. 13, 1971 Canada 95355 die wallan he web surface. As the web passes through the die the coatingmaterial between the web [52] US. Cl. 427/209; 118/404; 118/405; surfaceand the die wall is sheared and a coating of 427/434; 427/439 same isapplied to the web surface. The coated web is [51] Int. GL B05C 9/04;BOSC 3/02; BOSC 3/12 thereafter passed through a drying zone to effectthe [58] Field of Search 117/113, 115, 111 H, 68; required degree ofdrying.

14 Claims, 4 Drawing Figures FLOATER 15 DRYER FANS- BURNER US PatentSept. 23,1975

DRYER FLOATER 8 i Q Sheet 1 of 3 FANS- BURNER US Patent Sept. 23,1975Sheet 2 of3 3,908,044

US Patent Sept. 23,1975 Sheet 3 0f 3 3 ,908,044

1 APPARATUS AND METHOD FOR COATING A WEB CROSS REFERENCE TO RELATEDAPPLICATION This application is a continuation of application Ser. No.187,785, filed Oct. 8th, 1971, now abandoned This invention relates toimprovements in methods and apparatus for applying a coating material toan elongated web of material.

A need has existed for quite some time for improved coating devices andmethods for coating rapidly moving webs, as for example, a paper web.Known forms of coating devices generally employ some form of devicewhich contacts the moving web, as for example, roll coaters which employand applicator roll which rotates while in contact with the web to applycoating thereto, and blade type coaters arranged such that a pool ofcoating material is retained in a trough defined-by the blade and themoving web. Various problems exist with these methods e.g. the coatingapplied may possess an undersirable pattern characteristic caused bysplitting of the film of coating etc., the thickness of the coatingapplied is difficult to control, and it is difficult to apply a pin holefree in cases where a continuous barrier film is required, Most coatingdevices have moving parts which tend to wear and give trouble. Inaddition, since high rates of shear are involved, there are limitationsas to the types of coating materials which can be used. Changes in therheological properties of the coating material at high rates of shearmay cause problems.

The present invention provides ways and means for reducing to a largeextent many of the problems inherent in known forms of prior art webcoating devices particularly those problems ralating to coating patternsand rheological property changes. The invention provides a method and anapparatus for coating a moving web by passing the latter through anarrow coating die opening while maintaining a free flowing coatingmaterial between the a wall of the die opening and the web surface beingcoated. The coating material wets both said coating die wall and the websurface. As the web passes through the coating die the coating materialbetween the web surface and the coating die wall is sheared and acoating of same is applied to the web surface. The coated web isthereafter passed through a drying zone to effect the required degree ofdrying.

One form of device for applying a coating material to a moving web,which bears a superficial resemblance to the present invention, is shownin US. Pat. No. 2,766,721 to Paquette. The device is used to apply athixotropic slurry to a moving tape, the coated tape being used to makebattery electrodes. The apparatus used to define the coating dieincludes spaced apart side walls one of which slopes away from the planeof the moving tape by a small angle so as to create a wcdging actionwhich has the effect of compressing the coating material onto the websurfaces. Provision has to be made to continuously stir the thixotropicslurry to provide uniform consistency of same.

The present invention permits the application of films of coatingmaterial which are very thin, as compared to the coating thicknessesenvisaged by Paquette, to a web which may be moved at speeds many timeshigher than those which could be used with the paquette arrangement. Thepresent invention does not use a wcdging action to apply coating to theweb; rather the coating die is arranged and dimensioned in accordancewith the thickness of the moving web to provide a high rate of shear inthe free flowing coating liquid which wets both the surface and thecoating die wall. It is believed that this shearing action parallel tothe web surface is responsible for the substantial elimination of theundersirable coating pattern characteristics referred to above. A widevariety of coating liquids can be employed regardless of whether they beNewtonian, dilantant, thixotropic, pseudoplastic, etc. provided they wetperfectly the web surface and the orifice wall.

A specific embodiment of the invention will now be described withreference being made to the drawings wherein:

FIG. 1 is a schematic drawing of the coating device and its associatedequipment;

FIG. 2 is a plan view of the coating die and coating reservoir assemblythrough which the web passes;

FIG. 3 is a cross-section view of the coating die and coating reservoirassembly taken along line 33 in FIG. 2;

FIG. 4 is a cross-section view of a portion of the coating die, per setaken along line 4-4 in FIG. 2.

The coating device shown in FIG. 1 includes a supply reel 10 whichcontains a length of the web to be coated. Reel 10 is provided with asuitable brake arrangement (not shown) to maintain a suitable tension inthe web 14 from unwind to windup as slackness or flapping of the web cancause difficulties. Web 14 then passes over rolls l5 and over upper turnroll 16; the web then passes vertically downwardly to and around lowerturn roll 18. Web 14 then passes along to a take up reel 20, the latterpreferably being provided with a take-up drive 22. A suitable speedsensor (not shown) will be provided which will be operatively connectedto the take-up drive 22 to maintain a constant speed of the running webunder the possible changes in web tension brought about by variations inthe braking action on roll 10 and therefore the tension of the webentering the coating unit 26 and also changes in the drag on the webpassing through the coating unit 26 caused by changes in the viscosityof the liquid coating being applied, i.e. shearing forces. The speedselected is governed by the capacity of the dryer, the properties of theweb, etc., and is at the discretion of the operator. It has been foundand is theoretically confirmed that changes in viscosity of the coatingmaterial do not change the thickness of the film applied but do changethe tension in the web at any given speed due to resulting changes inthe shearing forces.

Disposed in the vertically downwardly extending run of web 14 betweenrolls l6 and 18 in a web coater 26 through which the web passes so thatthe coating material may be applied thereto. Below the web coater is aweb drying zone defined by a dryer 28 which effects the desired degreeof drying of the moving coated web.

With reference to FIGS. 2, 3 and 4 the web coater 26 includes reservoir29 for coating material which leads into a coating die 34 defined by apair of steel bars 30 disposed in side by side relation and spaced aprecise distance apart so that a' die opening is defined therebetween.The die opening is of generally elongated slotlike configuration and isdefined by a pair of spaced, parallel planar side walls 36 defined bythe sides of the spaced bars 30. The die opening has a length dimensionL slightly greater than the width of the web 14, a relatively smallwidth dimension W (measured normal to the planes of the opposing sidewalls 36 of the coating die) and a height dimension H (measured parallelto the direction of web travel). The flat side walls 36 of the coatingdie are perfectly parallel to one another and they are smoothlyfinished. Walls 36 should be hardened sufficiently as to preventabrasion thereof die to the shearing of certain particle containingcoating materials between the opposing surfaces of web 14 and theopposing coating die walls as will be more fully explained hereinafter.I

The steel bars 30 are bolted to the underside of the reservoir 29 forthe coating material. Reservoir 29 is defined by a pair of opposedmembers 37 and end walls 37' which together define a trough likereceptacle for the coating material. As seen in FIG. 3, each bar 30 isbolted to the underside of a respective one of the members 37 bythreaded studs 38 spaced along the length of each bar. Adjustment screws39 each having opposed left and right hand threaded portions areinterconnected between each bar 30 and its associated reservoir member37 at spaced intervals to provide for precise adjustments to be made inthe width (W) of the slat-like die opening. In a typical coating machinethe adjustment means should be designed to provide for adjustments fromabout 0.003 to about 0.030 inch with an accuracy of about 10.00025inches.

The bars 30 are each machined so that together they provide a hollowedout portion 40 leading from the reservoir 29 into the upper end ofcoating die 34. A suitable supply means 42 supplies the coating materialto the reservoir 29 to maintain a suitable level of this materialtherein at all times. The operating level will be somewhat above aminimum level required for operability. Itappears that the optimumcoating level will vary somewhat depeding on the exact coatingcomposition and the speed of the web. The longitudinally extending edges43 at the entrance of the upper end of the coating die opening are eachprovided with convex contours of relatively small but equal radiusthereby to generate streamline flow in the coating liquid from thereservoir 29 into the regions intermediate the web surfaces and thecoating die walls 36. The flow patterns set up positively center the web14 within the coating die opening providing any external forces appliedto the web surface are not too great. For this reason it is importantthat the web run between rolls 16 and 18 be adjusted so that it passesthrough the geometric center of the coating die opening. It can be shownthat the flow pattern into and within the die opening is such as toapply a greater force at right angles to the direction of web travel onthe side having the thinner liquid layer whereby the web isself-centering. Therefore, as the web 14 passes through the die opening,equal hydraulic forces are applied. to the opposing sides of the webcausing it to pass through the geometrical centre of the die openingthus causing the same amount of coating material to be applied to eachside of the web. At the web speeds contemplated, the hydraulic forcesencountered are relatively large.

Reservoir members 37 are shown as being relatively massive instructureas they serve tosupport elongated bars 30 and prevent sagging thereof inthe middle. This is particularly important in large machines where thelength of the bars 30 may exceed 150 inches. In like manner, the supportstructure for the whole coating die assembly should be strong and rigidto prevent undue vibration of the whole assembly. The support structuremay be designed such that one reservoir member 37 and its associated bar30 is movable away from the other to facilitate threading up of thecoating machine. After web 14 has been passed through between the twoassemblies, the movable reservoir member and bar assembly will bebrought forward and securely clamped or bolted to the reservoir memberand bar assembly in such a way that the pre-set die opening widthsetting is left unchanged. Shims of the required thickness may be placeat both ends of the die opening to obtain the desired spacing'betweenfaces 36.

' The speed movement of web 14 can be varied considerably. A typical webspeed would be around 1500 feet per minute; at present, the web speedscontemplated range from about 10 feet per minute to about 3000 feet perminute. In like manner the averagerate of shear of the coating materialmay be varied considerably. (The rate of' shear equals web velocitydivided by the distance from the surface of the centered web to thecoating die wall next adjacent thereto.) A typical rate of shear isabout 300,000 reciprocal seconds (sec"). However, satisfactory operationof the apparatus can be expected at shear rates anywhere from about 1000see to about 600,000 sec. There is no apparent limit to the width of thewebs which can be coated and web widths of 100, 150, 200 inches etc. arevisualized The thickness of the coatings applied can be variedconsiderably. The preferred thickness vary from about 0.00025 to about0.003 inches in their dried state. The 0.00025 inch thick coatings can,for example, be laid down from about a 10% solids content coatingsolution; in this case the wet film thickness would be 10 X 0.00025 orabout 0.0025 inches thick. Pigmented films are generally laid down fromsolutions with suspended solids and/or in liquid emulsions withsuspended solids that are about 40% water so that a dry pigmented film0.0015 inch thick will result from a wet film thickness of about 0.0025inches. The width W of the die opening must be very accurately chosen ifthe desired film thicknesses are to achieved since the film thicknesswill equal the distance between the surface of the centered web and thecoating die wall. In the embodiment shown, the adjustment means 39 isused to achieve the desired die opening with. Once this has been set nofurther adjustment is required provided the solids content etc. of thecoating solution is kept at a constant value.

In general, the maximum die opening with dimension (W) will be equal totwice the wet film thickness plus the web thickness. For practicalpurpose (W) will range from about 0.003 inch to about 0.020 inch. The0.003 inch dimension could be used in a typical coating operation forpreparing light packaging foils wherein the web is about 0.001 inch withspacings of 0.001 being maintained between each of the web surfaces andthe coating die adjacent thereto. The 0.020 dimension could be used whencoating a heavy paperboard about 0.14 inches thick with a heavy coatinghaving a wet film thickness of about 0.003 inches. The requireddimension (W) is thus 0.014 +(2 X 0.003) 0.020 inch. For ordinaryprinting papers the wet film thickness may also be about 0.003 inchesthus requiring a die opening width of about 2 X 0.003 (paper thickness).Since these paperscommonly have thicknesses ranging from about 0.002 toabout 0.005 inches, the required die opening width will usually besomewhere in the range of 0.008 to about 0.011 inches.

The viscosity of the coating materials used may also be variedconsiderably providing the coating alway sremains free flowing asopposed to being paste like and is capable of wetting the web andcoating die wall surface.

It has been found that there is no need to provide separateagitatingdevices for the coating liquid which is in the reservoir 29.Since the liquid wets the downwardly moving web, a rolling motion is setup within the reservoir which ensures good mixing. In fact, guards maybe necessary to prevent the coating liquid from splashing out of thereservoir at high web speed rates.

The dryer 28 is, per se, of a type wellknown in the art and must becapable of effecting substantial drying of the coated web before itreaches the lower turn roll 18; otherwise some of the coating may tendto come off the web and build up on roll 18 with unfortunate results.Roll 18 could be a Levitron roll (suppled in Canada by Devtek Limited,Toronto) which maintains as air film between the roll face and the websurface. Since physical contact of the web with the roll is avoided, theproblem of the web coating adhering to the roll is avoided. Since theweb is wound up after passing around turn roll 18, it is apparent thatin all cases a substantial amount of web drying will be required toavoid any sticking of the layers of wound up web together. The dryer mayincorporate any suitable heating means e.g. banks of electric heatingelements, gas fired heaters or high velocity hot air which impinges onthe web. The required rate of heat output and the length of the dryerstage will vary according to the desired speed of web travel and thenature of the coating. Those skilled in the art-will of course bereadily able to make the required determinations regarding the dryer forany particular machine setup.

In operation,.as the web passes downwardly through reservoir 29, itssurfaces are wetted by the coating material. As the web moves downthrough coating die 34 the liquid between the web surfaces and theopposing coating die walls 36 of the die opening (which are also wettedby the coating material) is shearedlso that viscous drag forces areexerted on the moving web. Since the coating material wets the coatingdie walls there is no relative movement between said walls and theliquid layers adjacent thereto. In like manner there is no relativemovement between the liquid layers adjacent the opposing surfaces of theweb 14. Hence it follows that there is relative movement between theremaining various portions or layers of liquid disposed between thecoating die walls 36 and the web surfaces; this shearing of the coatingmaterial in the planar regions between the coating die walls 36 and theweb surfaces parallel thereto is responsible in large measure for theexcellent uniformity and continuity of the coatings which may be appliedto the web surfaces. Since there is no relative movement between walls36 and the adjacent liquid, the need for hard metal wall surfaces toprevent abrasion may be questioned. However, the answer appears to bethat the abrasion is a result of the presence of solid particles in theliquid which are acted upon (dragged) by the liquid which is disposedoutwardly of coating die walls 36; the resulting movement of these solidparticles probably causes them to pierce the static layers adjacentwalls 36 and produce the abrasion proviously referred to.

The principles of the invention will be further under;

stood from the following typical examples:

EXAMPLE 1 Web material paper Web thickness .0028 inches Web speed l5 ft.per minute Die opening width (W) .008 inches Clearance between surfaceof centered web and die wall ,0026 inches Rate of shear l 155 secCoating material 107: solution of polyvinyl alcohol Viscosity of coatingmaterial I000 cps at 20 rpm (Brookfield) The basis weight of the paperused was 3 1 lb. per ream, 25 X 38 500, and the basis weight of thefinished coated paper was 38 lb. The coat weight per side was therefore3% lb. per ream. The standard TAPPI test with dyed turpentine showed nopinholes and complete resistance to the test liquid.

EXAMPLE II Web material paper Web thickness .0025 inches Web speed 10ft. per minute Die opening width (W) .0067 inches Clearance betweensurface of centered web and die wall .004 inches Rate of shear I 900 secCoating material standard printing coating 54% solids Viscosity ofcoating material 200 cps (Brookfield) The basis weight of the uncoatedpaper was 37.4 lb. per ream 25 X 38 500. After coating the basis weightwas 60.3 lb., so that the total coating applied was 22.9 lb. per ream or1 1.4 lb. per side. The coating material was a standard printing papercoating furnish containing Kaolin and binder at 54% solids of which 18%was adhesive,

EXAMPLE III Web material Mylar" (Reg. T.M.)v Web thickness .001 inchesWeb speed 7 1500 ft. per minute Die opening width (W) .003 inchesClearance between surfaceof centered web and die.wall .001 inches Rateof shear 300.000 sec Coating material standard printing coating 54%solids as in Example II 200 cps (Brookfield viscosity) Viscosity ofcoating material The applied coating was smooth and uniform and wasjudged satisfactory in all respects.

In both Examplesil and II the web speeds were limited because ofinadequate dryer capacity on the experimental apparatus used. ExampleIII illustrates that web speeds many times greater than those used inExamples I and II are possible:

Several of the advantages of the arrangement described are given below.

I. There are no moving parts in the web coater; only the web moves. I

2. The coating on each surface is formed by a shearing action and so isfree of all rheological patterns characteristics of other methods.

3. The wet coating thickness can be controlled by adjusting the distancebetween the coating die walls, and once set no further adjustment isneeded provided certain physical characteristics of the coatingmaterial, particularly the solids content of the coating material, arecontrolled.

4. A very wide range of coating color solids can be used, as forexample, the various china clay (Kaolin) adhesive mixtures used forcoating printing papers, var ious greaseproof coatings for packagingfoils such solutions of polyvinyl alcohol or polyvinyl acetate and so onso long as the coating die walls and the web surfaces are wetted therebyto produce the required shearing action in the coating material parallelto the web surface being coated,

5. A very wide range of coating thicknesses can be applied.

6. Higher coating solids with less water or other solvent can be used sothat the amount of drying is minimized.

7. Pin-holing is substantially eliminated so that continuous barriercoatings can be applied in one pass. This is particularly illustrated byExample I.

8. The web moves downward in a vertical plane so that both sides can bedried before the surface need touch rollers or other means of handling aweb.

9. Very high rates of web speed are possible.

10. The apparatus can easily be operated to give constant filmthicknesses even with variations in viscosity. In the present inventionthe film thickness is set me chanically and so as long as the solidscontent of the coating mixture is constant, it does not matter what theviscosity of the coating is as long as the force which must be appliedto the web to draw it through the coating die does not exceed that whichthe web is capable of withstanding.

I claim:

1. A method of coating a flexible web comprising the following steps:

a. passing the web through reservoir means containing a liquid coatingmaterial for wetting the web surface by the coating material;

b. subsequent to step (a), passing the web through a die opening definedby spaced die walls in communication with the reservoir means for beingwetted by the coating material;

c. throughout the coating operation, maintaining the die walls alongtheir entire extent in perfect parallelism with respect to one anotherand with a spacing having a predetermined value ranging in magnitudefrom about 0.003 inch to about 0.020 inch;

d. as a part of step (b), maintaining the web at a predetermined,parallel-spaced relationship throughout substantially the entire extentof the die walls for effecting coating by shearing the coating materialin contact with the web surface in a direction parallel thereto, whileavoiding wedging of the coating material between the die walls and theweb; and

e. subsequent to steps (b) and (d), passing the coated web through adrying zone.

2. The method according to claim 1, further including the step ofmaintaining said reservoir means with the coating material immediatelyadjacent the entrance to said die opening thus generating flow of thecoating liquid from the reservoir means into the regions intermediatethe web surfaces and the walls of the coating die, which flows ofmaterial into said regions generate hydraulic forces tending to centerthe web between the walls of the die opening to provide for equalthicknesses of coating to be applied to both surfaces of said web.

of shear of said coating material is from about 1000 to about 600,000sec".

5. The method according to claim 1 wherein the web comprises paper.

6. The method according to claim 2 wherein the rate of shear of saidcoating material is not greater than about 600,000 sec.

7. Apparatus for coating a flexible web advanced through the apparatus,comprising:

a. reservoir means for accommodating a liquid coating material; b. acoating die connected to said reservoir means;

said coating die including 1. means defining a die opening constitutedsolely by a pair of die walls extending in perfect parallelism to oneanother and having a predetermined spacing ranging in magnitude fromabout 0.003 inch to about 0.020 inch; said walls being exposed to thecoating material in said reservoir means;

2. means for maintaining unchanged said predetermined spacing during thecoating operation;

c. drier means situated downstream of said coating die with respect tothe direction of web advance for removing moisture from the coated web;and

d. guiding means for passing the web in a path of travel in successionthrough said reservoir means for wetting the web surface with thecoating material, through said die opening and through said drier means;said guiding means causing the web to pass in a predetermined spacedrelationship to said walls in parallelism thereto throughoutsubstantially the entire extent of said walls to effect coating of saidweb by shearing of the coating material by said walls.

8. Apparatus as defined in claim 1, including adjusting means forsetting said predetermined spacing to a selected magnitude.

9. Apparatus according to claim 7, said coating die being arrangedrelative to the guiding means for passing the web in the path of travelsuch that the web passes intermediate the space apart walls inparallelism thereto, said reservoir means being disposed immediately inadvance of the die opening through which reservoir the web passes thusgenerating flow of the coating liquid from the reservoir into theregions intermediate the web surfaces and the parallel walls of the coating die which flows of material generate hydraulic forces tending toeffect centering of the web being the parallel walls of the die wherebyto apply equal coating thicknesses to both of the opposed web surfaces.

10. Apparatus according to claim 7, further including means for movingthe web at a speed such, and wherein the die walls are spaced from theweb surface sufficiently that, a rate of shear of said coating materialfrom about 1000 to about 600,000 sec is produced.

1 1. Apparatus according to claim 7 wherein the guiding means forpassing the web is arranged such that its path of travel is directedvertically downwardly.

12. Apparatus according to claim 7 wherein the die walls are made of ahard, abrasion resistant material.

13. Apparatus according to claim 7 wherein the die walls are flat orplanar.

14. Apparatus according to claim 7 wherein the distance between said diewalls is from about 0.008 to about 0.01 1 inch.

3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 13,908,044

DATED September 23rd, 1975 INVENTOR(S) John R. Gunning It is certifiedthat error appears in the above-identified patent and that said LettersPatent Q are hereby corrected as shown below:

In theheading of the patent, under [75] Inventor, after "Oakville"insert -Ontario; under [30] Foreign Application Priority Data, change"Oct. 13, 1971" to Oct. 13, 1970-- Column 1, line 6, insert a periodafter "abandoned"; line 15 change "and" to -an-; line 24 after "free"insert fi1m;

line 35, change "ralating" to -re1ating; line 40, after "between" delete"the"; lines 65-66, change "paquette" to Paquette-; line 67 put quotesaround "wedging" Column 2, line 4, before "surface" insert web; line 32,change "take up" to take-up-; line 34, after "suitable" insert web; line52 change "in" to -is.

Column 3, line 6, change "die" to --due; line 23, change "slat-like" to-s1otlike-;line 35, change "depeding" to depending-; line 37, change"of" to -to; line 44, after "applied" insert --norma1-; line 55, change"centre" to center-. i

Column 4, line 10, change "place" to placed--; line 27, change"thickness" to thicknesses--; line 28, change "thickness" to-thicknesses-; line 35, after "and/or" insert -liquid;

line 41, after "to" insert beand after "the" insert -wet; line 45,change "with" to width-; line 50, change "purpose" to purposes; line 54after "inch" insert thick; line 56, after "die" insert -wa11-; line 58change "0.14" to 0.014-. Column 5, line 18, change "suppled" tosupplied-; line 20, change "as" to --an; line 65, change "proviously" to--previously-.

Column 6 line 61, change "characteristics" to --characteristic- Column8, line 38, change "1" to --7-; line 44, change "space apart" to---'-spacedapart; line 50, change "being" to between-.

Signed and Sealed this seventeenth Day of February 1976 [SEAL] A ttest:

RUTH C. MASON C.MARSHALL DANN Arresting Officer (ummissimrer oflarenrsand Trademarks

1. A METHOD OF COATING A FLEXIBLE WEB COMPRISING THE FOLLOWING STEPS: A.PASSING THE WEB THROUGH RESERVOIR MEANS CONTAINING A LIQUID COATINGMATERIAL FOR WETTING THE WEB SURFACE BY THE COATING MATERIAL, B.SUBSEQUENT TO STEP (A), PASSING THE WEB THROUGH A DIE OPENING DEFINED BYSPACED DIE WALLS IN COMMUNICATION WITH THE RESERVOIR MEANS FOR BEINGWETTED BY THE COATING MATERIAL, C. THROUGHOUT THE COATING OPERATION,MAINTAINING THE DIE WALLS ALONG THEIR ENTIRE EXTEND IN PERFECTPARALLELISM WITH RESPECT TO ONE ANOTHER AND WITH A SPACING HAVING APREDETERMINED VALUE RANGING IN MAGNITUDE FROM ABOUT 0.003 INCH TO ABOUT0.020 INCH, D. AS A PART OF STEP (B), MAINTAINING THE WEB AT APREDETERMINED, PARALLEL-SPACED RELATIONSHIP THROUGHOUT SUBSTANTIALLY THEENTIRE EXTENT OF THE DIE WALLS FOR EFFECTING COATING BY SHEARING THECOATING MATERIAL IN CONTACT WITH THE WEB SURFACE IN A DIRECTION PARALLELTHERETO, WHILE AVOIDING WEDGING OF THE COATING MATERIAL BETWEEN THE DIEWALLS AND THE WEB, AND C. SUBSEQUENT TO STEPS (B) AND (D), PASSING THECOATED WEB THROUGH A DRYING ZONE.
 2. means for maintaining unchangedsaid predetermined spacing during the coating operation; c. drier meanssituated downstream of said coating die with respect to the direction ofweb advance for removing moisture from the coated web; and d. guidingmeans for passing the web in a path of travel in succession through saidreservoir means for wetting the web surface with the coating material,through said die opening and through said drier means; said guidingmeans causing the web to pass in a predetermined spaced relationship tosaid walls in parallelism thereto throughout substantially the entireextent of said walls to effect coating of said web by shearing of thecoating material by said walls.
 2. The method according to claim 1,further including the step of maintaining said reservoir means with thecoating material immediately adjacent the entrance to said die openingthus generating flow of the coating liquid from the reservoir means intothe regions intermediate the web surfaces and the walls of the coatingdie, which flows of material into said regions generate hydraulic forcestending to center the web between the walls of the die opening toprovide for equal thicknesses of coating to be applied to both surfacesof said web.
 3. The method according to claim 2 wherein said walls areplanar.
 4. The method according to claim 1 wherein the rate of shear ofsaid coating material is from about 1000 to about 600,000 sec
 1. 5. Themethod according to claim 1 wherein the web comprises paper.
 6. Themethod according to claim 2 wherein the rate of shear of said coatingmaterial is not greater than about 600,000 sec
 1. 7. Apparatus forcoating a flexible web advanced through the apparatus, comprising: a.reservoir means for accommodating a liquid coating material; b. acoating die connected to said reservoir means; said coating dieincluding
 8. Apparatus as defined in claim 1, including adjusting meansfor setting said predetermined spacing to a selected magnitude. 9.Apparatus according to claim 7, said coating die being arranged relativeto the guiding means for passing the web in the path of travel such thatthe web passes intermediate the space apart walls in parallelismthereto, said reservoir means being disposed immediately in advance ofthe die opening through which reservoir the web passes thus generatingflow of the coating liquid from the reservoir into the regionsintermediate the web surfaces and the parallel walls of the coating diewhich flows of material generate hydraulic forces tending to effectcentering of the web being the parallel walls of the die whereby toapply equal coating thicknesses to both of the opposed web surfaces. 10.Apparatus according to claim 7, further including means for moving theweb at a speed such, and wherein the die walls are spaced from the websurface sufficiently that, a rate of shear of said coating material fromabout 1000 to about 600,000 sec 1 is produced.
 11. Apparatus accordingto claim 7 wherein the guiding means for passing the web is arrangedsuch that its path of travel is directed verticaLly downwardly. 12.Apparatus according to claim 7 wherein the die walls are made of a hard,abrasion resistant material.
 13. Apparatus according to claim 7 whereinthe die walls are flat or planar.
 14. Apparatus according to claim 7wherein the distance between said die walls is from about 0.008 to about0.011 inch.